This invention relates generally to steam desuperheaters and more particularly to a spray desuperheater for spraying water into a steam pipe to maintain the steam at a predetermined temperature level.
Many applications using steam operate most efficiently using steam that is saturated or slightly superheated, while many steam generators such as boilers tend to produce steam that is sometimes excessively superheated. This is particularly true where the steam demand of the application changes more rapidly than the output response of the steam generator. Under these conditions, the optimum efficiency can be obtained by powering the steam generator to produce superheated steam, and then reducing the amount of superheat by injecting water into the steam.
A common type of water injection uses a spray head placed at the middle of the steam pipe and having a fixed spray nozzle adapted to produce a fine spray of water downstream into the flowing steam. The actual volume of water injected into the steam is therefore varied by changing the pressure of the water supply, and the pressure must always be maintained well above that of the steam in the line at the nozzle. Because the amount of superheat varies with the amount of steam production and the rate of flow, among other variables, the only true regulation can be made by sensing the temperature of the steam at a point downstream where the injected water has been completely vaporized and heated so that equilibrium conditions have been reached. This requires a spray nozzle that is optimized for a certain rate of flow, and if the flow rate is varied outside of certain parameters, the resulting spray pattern may not give prompt enough heat transfer to allow an equilibrium condition to be sensed and proper steam conditions be attained. However, such desuperheaters work well when the spray requirements vary over only a narrow range.
When a greater variation in spray volume is required, other types have been used, including variable orifices and the use of a separate steam path for premixing with the water flow. Another arrangement has been the use of multiple nozzles with a moving plug or slide member uncovering or selecting different nozzles for spraying the water. One such arrangement which has proved successful has been a multiple nozzle spray unit disclosed in U.S. Pat. No. 4,442,047, owned by the assignee of this application. This unit uses a spray tube extending into the steam line and has a plurality of small nozzles on the downstream side spaced varying distances from the end. The nozzles are connected to a bore and a hollow plug is moved to and from the end to uncover different numbers of nozzles to vary the volume of water being sprayed. The position of the plug is determined by a valve stem which is moved linearly by a diaphragm actuator which in turn is controlled in response to signals from a temperature sensor located downstream in the steam line. The complexity of the actuating system results in high original cost as well as high maintenance after installation.